1. Field of the Invention:
This invention relates to a method for improving mechanical and chemical properties of styrene-butadiene rubber (SBR) and the improved rubber produced thereby.
2. Background of the Prior Art:
SBR is well known as a synthetic alternative to natural rubber (cispolyisoprene) developed in response to rubber shortages encountered during World War II. To improve physical characteristics for many applications, such as tires and the like, specific and selective curing processes have been employed with SBR. Two processes currently tend to dominate the market place.
In a first process, the SBR is modified by the addition of a chemical improver, generally sulfur, and the rubber is cured in the presence of the improver and moderate heat. This is the traditional method of curing SBR for industrial applications such as tires, hoses and the like.
An alternative method begins with limited or "light" irradiation of the polymer, followed by the chemical/thermal curing described above. This process is generally used in the production of components of tires for automobiles, etc. In this process, the initial light irradiation is used to produce a limited crosslink density, which increases "green strength" sufficiently, such that the irradiated preform can withstand the handling in the subsequent tire fabrication process. The major amount of crosslinking, and therefore, curing of the SBR is achieved through the second, heat/sulfur cure stage.
A third class of alternatives includes some variations, either in chemical additives, heating, or irradiation. These combinations and permutations are generally selected with regard to the ultimate end-use of the elastomer, and the particular nature of the elastomer (i.e., styrene content, etc.)
Neither of the dominant methods, nor any known combinations, have been employed in the modification of SBR for the purpose of improving tear strength or ozone resistance.
NR is the focus of U.S. Pat. No. 2,933,441. Therein, a combination of processes, beginning with an accelerated heat cure, using sulfur and a small amount of accelerator, is the first step in the production of a tire. The partially cured preform is treated so as to retain its shape under subsequent irradiation and handling treatments. Thereafter, it is irradiated under an intense, localized beam. The reference discloses such a treatment improves abrasion resistance. It does not discuss ozone resistance or tear strength.
Other references specifically directed to SBR, as opposed to the NR of the reference discussed above, include U.S. Pat. Nos. 4,122,137; 4,230,649 and 4,233,013 all to Bohm et al. These references are directed to the curing of SBR, through a sulfur/heat treatment, followed by irradiation. As suggested in Example 3 of U.S. Pat. No. 4,122,137, the process calls for a relatively large amount of accelerator (1.8 phr) and a reduced irradiation dosage. U.S. Pat. No. 4,230,649 differs from that disclosure principally in using a microwave irradiation step subsequent to the sulfur cure. These patents are all directed to improving the processing efficiency, i.e., reducing in-mold time, and do not focus on the improvement of specific characteristics, such as ozone resistance and hot tear strength. SBR is frequently employed in situations where improved overall wear resistance, tear strength, crack initiation and growth resistance and, in particular, ozone resistance, are of importance, such as in pads for military track vehicles, and the like. In the particular situation of pads for track vehicles, the pads must be of a compound resistant to constant wear over asphalt/concrete surfaces, and cracking and pitting over hilly or cross-country terrain. Suitable materials are routinely evaluated by the military in field tests. Thus, a need continues to exist for a method for improving the mechanical strengths and ozone resistance of SBR for such applications.